Biomass torrefaction system and method

ABSTRACT

A biomass torrefaction system is provided which enables a continuous torrefaction process that involves the introduction of biomass particles into a rotating reactor drum having a low oxygen environment. The particles are conveyed through the drum by a heated gas stream and simultaneously torrefied thereby. Gas exiting the drum is recirculated back to a heat source for reheating the gas prior to reentering the drum. A method of biomass torrefaction is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of InternationalApplication No. PCT/US2011/055153, accorded an International Filing Dateof Oct. 6, 2011, which claims the benefit of the filing date of U.S.Provisional Application No. 61/391,442, filed Oct. 8, 2010 and U.S.patent application Ser. No. 13/218,230, filed Aug. 25, 2011, the entirecontents of which are incorporated herein by reference for all purposes.

BACKGROUND

1. Technical Field

This disclosure generally relates to biomass torrefaction systems andmethods, including in particular cellulosic biomass torrefaction systemsand methods.

2. Description of the Related Art

Torrefaction of biomass particles is well known and is a process inwhich biomass particles are heated in a low oxygen environment. Thiscauses volatile compounds within the particles to be boiled off and thecellular structure of the particles to be degraded, resulting in apartial loss of mass and an increase in friability. It also causes areaction within the remaining cellular structure that enhances themoisture resistance of the product. Torrefied particles have an enhancedenergy value when measured in terms of heat energy per unit of weight.The degree of torrefaction of biomass particles depends on severalfactors, including the level of heat applied, the length of time theheat is applied, and surrounding gas conditions (particularly withrespect to oxygen level).

Current systems strive to mechanically control the variables of heat,residence time and oxygen levels to achieve consistent torrefiedparticles. Typical mechanisms intended to torrefy biomass particlesunder low level oxygen conditions use mechanical means to convey theparticles (such as rotating trays or screws) and apply heat to theconveying surfaces for conduction to the particles to be torrefied. Suchmechanisms suffer from a variety of drawbacks, including being difficultor impossible to significantly scale up in capacity. As the demand fortorrefied biomass increases, the limited capacity of current mechanismshas become an issue impeding the use of such biomass. Consequently,Applicant believes improved methods and systems able to consistently andefficiently produce torrefied biomass particles are desirable. Thesemethods and systems should be based on principles and concepts thatallow tight process control while achieving large capacities, to meetgrowing demand.

BRIEF SUMMARY

Embodiments described herein provide biomass torrefaction systems andmethods which are particularly well adapted for torrefying biomassparticles (including in particular cellulosic biomass particles) ofvarious sizes in an efficient and consistent manner. The systems andmethods are readily scalable to meet a wide variety of industry needsand provide enhanced process control with respect to monitoring andadjusting operational parameters to optimize or tailor characteristicsof the resultant torrefied biomass particles.

According to one embodiment, a biomass torrefaction system may besummarized as including an inlet to receive biomass particles; a reactordrum configured to rotate about its longitudinal axis, the reactor drumhaving a plurality of flights positioned therein at a plurality oflocations along the length of the reactor drum; a heat source upstreamof the reactor drum to heat gas contained in the system to a temperaturesufficient to torrefy the biomass particles during operation; a fandevice coupled to the system to create, when the system is in operation,a flow of heated gas through the reactor drum sufficient tointermittently transport the biomass particles along the length of thereactor drum as the biomass particles are lifted by the flights andshowered through the heated gas stream as the reactor drum rotates; andgas ducts coupled to at least the reactor drum, heat source and fandevice to recirculate a portion of gas exiting the reactor drum back tothe heat source to reheat the gas for reintroduction into the reactordrum.

The heated gas stream directly heats the biomass particles as the gasstream intermittently transports the biomass particles through thereactor drum. The lifting flights may be configured to regulate movementof the biomass particles through the reactor drum, thereby influencingthe retention time of the biomass particles within the reactor drum. Thelifting flights may include flights spaced around an inner circumferenceof the reactor drum in regular or irregular intervals and in at leastthree locations along the longitudinal length of the reactor drum. Thelifting flights interoperate with the heated gas stream to classify thebiomass particles according to particle density and/or size, by movingrelatively denser particles with respect to similarly sized particlesand relatively larger particles with respect to particles having similardensities through the reactor drum more slowly.

The biomass torrefaction system may further include a hopper locateddownstream of the reactor drum to collect torrefied biomass particlesexiting the reactor drum and to discharge the torrefied biomassparticles from the system. The system may further include ducting todispel exhaust gas from the system, with control valves and dampers, thecontrol valves and dampers positioned to regulate a pressure levelwithin the system to inhibit the infiltration of oxygen while enablingexhaust gas to exit the system. The ducting may route exhaust gas fromthe system to a remote device for use of the exhaust gas in an auxiliaryor supplemental process. The remote device may be, for example, a burnerconfigured to utilize the exhaust gas for supplying heat via a heatexchanger to the gas which passes through the reactor drum duringoperation.

The system may further include at least one airlock located between theinlet and the reactor drum to limit the amount of oxygen entering thesystem when receiving the biomass particles. The system may furtherinclude at least one seal mechanism between the reactor drum andadjacent structures, the seal mechanism including a chamber between thereactor drum and an external environment and the seal mechanism coupledto an inert or semi-inert gas source for selective purging of thechamber during a startup or shutdown operation.

The heat source for the system may be an electrical immersion-type ductheater, gas-to-gas heat exchanger, a low-oxygen burner or otherconventional heat sources, such as, for example, a waste-wood or otherburner which is configured to supply heat indirectly to the gas streamin the biomass torrefaction system.

The biomass torrefaction system may further include a steam plantcoupled to the reactor drum to introduce steam into the reactor drum andassist in the torrefaction of the biomass particles. The steam plant mayalso provide safety smothering and cooling stream functionalities toenhance operational safety.

The biomass torrefaction system may further include a control systemconfigured to selectively adjust the speed of the fan device to regulatethe speed and volume of gas through the system. The control system mayalso be configured to selectively adjust the speed of the rotation ofthe reactor drum to regulate a time of residence of the biomassparticles in the reactor drum. The control system may also be configuredto selectively adjust the temperature of the flow of gas through thesystem. The control system may be configured to selectively adjustparameters of the flow of gas through the system including volume, speedand/or pressure. The control system may also be configured toindependently control a plurality of operational parameters to regulatea torrefaction process of the biomass particles, the operationalparameters including at least one of a reactor inlet temperature, areactor outlet temperature, an average residence time, oxygen content ofthe heated gas stream and gas flow characteristics. The control systemmay be configured to continuously or intermittingly adjust at least someof the operational parameters during operation to optimize thetorrefaction process or tailor characteristics of the resultanttorrefied biomass particles.

According to one embodiment, a method of biomass torrefaction may besummarized as including rotating a reactor drum, the reactor drum havinga plurality of flights positioned therein at each of a plurality oflocations along a longitudinal length of the reactor drum; generating astream of heated gas through the reactor drum, sufficient tointermittently transport biomass particles along the length of thereactor drum, and simultaneously torrefy the biomass particles as thebiomass particles are lifted by the flights and showered through theheated gas stream while the reactor drum rotates; and recirculating aportion of gas exiting the reactor drum back to the inlet of the reactordrum via one or more gas ducts.

The method may further include selectively varying at least some of aplurality of operational parameters to tailor characteristics of theresultant torrefied biomass particles, the operational parametersincluding at least one of a speed of the heated gas stream through thereactor drum, a volumetric flow rate of the heated gas stream throughthe reactor drum, a temperature of the heated gas stream through thereactor, a pressure level within the reactor drum, a speed of therotation of the reactor drum, oxygen content of the heated gas stream, amoisture content of the biomass particles and a rate of introduction ofthe biomass particles into the reactor drum. The method may furtherinclude selectively varying the time of residence of the biomassparticles in the reactor drum. The method may further include adjustingthe plurality of flights within the reactor drum with respect tolocation and/or density to regulate the retention time of the biomassparticles within the reactor drum. The method may further includepassing biomass particles through the reactor drum at different ratesaccording to particle density and/or size. The method may furtherinclude discharging torrefied biomass particles while substantiallypreventing the infiltration of oxygen into the reactor drum. The methodmay further include establishing a pressure level within the reactordrum to inhibit the infiltration of oxygen into the reactor drum. Themethod may further include routing exhaust gas to a device remote fromthe reactor drum for use of the exhaust gas in an auxiliary orsupplemental process, such as, for example, use as a fuel for a remoteburner.

The method may further include sealing the reactor drum from theexternal environment and selectively purging one or more chambersadjacent to sealing interfaces of the reactor drum with inert orsemi-inert gas. The method may further include passing biomass particlesthrough the reactor drum at a rate between about one to fifty tons perhour, the biomass particles having an energy density of at least 20gigajoules/ton (GJ/ton) after being torrefied within the reactor drum.

The method may further include drying the biomass particles in a rotarytype, conveyor type or other type of dryer system prior to introductionin the reactor drum. Drying biomass particles in the rotary type dryersystem prior to introduction in the reactor drum may include drying thebiomass particles to have an average moisture content below twentypercent moisture content, wet-weight basis.

The method may further include establishing the heated gas stream suchthat an inlet temperature of the heated gas stream entering the reactordrum is at least 500° F. and such that an outlet temperature of theheated gas stream exiting the reactor drum is at least 400° F. Themethod may further include discharging torrefied biomass particles aftera single pass of the biomass particles through the reactor drum,particle sizes of the discharged torrefied biomass particles varying byat least ten percent while the energy density and moisturecharacteristics of the torrefied biomass particles are relativelyconsistent irrespective of particle size. The method may further includeintroducing the biomass particles into the drum reactor, the biomassparticles having an average size of about 1/16 cubic inch to about onecubic inch upon entry. The method may further include venting thereactor drum upon a fault condition. The method may further includeintroducing steam into the reactor drum to assist in the torrefaction ofthe biomass particles. Introducing steam into the reactor drum mayinclude producing steam with a boiler which receives heat from a portionof gas exiting the reactor drum.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a biomass torrefaction system accordingto one embodiment.

FIG. 2 is a schematic diagram of an integrated biomass processing systemaccording to one embodiment.

FIG. 3 is a isometric view of a biomass torrefaction system according toanother embodiment.

FIG. 4 is a rear isometric view of the biomass torrefaction system ofFIG. 3.

FIG. 5 is a side elevational view of the biomass torrefaction system ofFIG. 3.

FIG. 6 is a top plan view of the biomass torrefaction system of FIG. 3.

FIG. 7 is a side elevational view of a reactor drum and adjacentcomponents of the biomass torrefaction system of FIG. 3.

FIG. 8 is a cross-sectional view of the reactor drum of FIG. 7 takenalong line 8-8.

FIG. 9 is a side elevational view of a seal assembly, according to oneembodiment, that is usable with the biomass torrefaction system of FIG.3.

FIG. 10 is an enlarged detail view of a portion of the seal assembly ofFIG. 9.

FIG. 11 is a cross-sectional view of the seal assembly of FIG. 9 takenalong line 11-11 of FIG. 10.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails. In other instances, well-known structures or steps associatedwith industrial process equipment and industrial processes may not beshown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments. For instance, it will be appreciated bythose of ordinary skill in the relevant art that various sensors (e.g.,temperature sensors, oxygen sensors, etc.), control devices and otherindustrial process controls may be provided and managed via aprogrammable logic controller (PLC) or other suitable control system formonitoring the biomass torrefaction systems described herein andcontrolling operational parameters of the torrefaction processes tooptimize or tailor characteristics of the resultant torrefied biomassparticles.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

FIG. 1 shows a schematic of a biomass torrefaction system 10 accordingto one example embodiment. The system 10 includes a reactor drum 12which is supported so as to rotate its longitudinal axis 16. The system10 further includes an inlet 22 for receiving biomass particles that areto be processed, as represented by the arrow labeled 24. An airlock ordual airlock 26 with optional inert or semi-inert gas purging 27 orsimilar device is coupled to the inlet 22 to substantially preventoxygen from entering the system 10 when biomass particles are fed intothe system 10. The biomass particles may be fed to the inlet 22 via aconveyor or other conventional material transport mechanism. In oneembodiment, a plug-feed screw conveyor may be used in lieu of theairlock(s) to create a plug of material that acts as a seal when passingbiomass particles through the inlet 22.

The system 10 further includes a heat source 30 disposed upstream of thereactor drum 12 for supplying heat to a gas stream 34 that is generatedwithin the system 10 by a fan device 32, which may be, for example, aninduced draft fan device or a forced draft fan device. The fan device 32is driven to draw or force gas through the reactor drum 12 and circulatethe gas (or a substantial portion of the gas) back to the heat source 30to be reheated and supplied to the reactor drum 12 in a recirculatingmanner. In some embodiments, eighty percent or more of the gas by volumeexiting the reactor drum 12 may be recirculated to the inlet of thereactor drum 12. In some embodiments, ninety percent or more of the gasby volume exiting the reactor drum 12 is recirculated to the inlet ofthe reactor drum 12. In some embodiments, ninety-five percent or more ofthe gas by volume exiting the reactor drum 12 is recirculated to theinlet of the reactor drum 12.

During operation, the gas stream 34 acts as a thermal fluid to carryheat energy to the biomass particles within the reactor drum 12 and toprovide momentum for conveyance of the biomass particles. The gas streammay also heat the internal structure of the drum 12, especially thelifting flights, which may also in turn heat the biomass particles. Gasducts 36 are appropriately sized and coupled to at least the reactordrum 12, heat source 30 and fan device 32 for recirculating the gasstream 34 in the system 10. In some embodiments, a predominate portionor the entire amount of gas entering the reactor drum 12 is recirculatedback to the inlet of the reactor drum 12 in a continuous manner while anamount of gas generated by torrefying the biomass particles is exhaustedor otherwise routed external the system 10. In some embodiments, no newgas (other than unintended leakage) is supplied to the recirculating gasstream 34 during operation.

In the illustrated embodiment, the heat source 30 is in the form of agas-to-gas heat exchanger 60. A hot gas stream 35, in the range of about800° F. to about 1400° F., for example, is supplied to the heatexchanger 60 via an inlet conduit 62, as represented by the arrowlabeled 64. The hot gas stream 35 interacts with the recirculating gasstream 34 of the torrefaction system 10 to transfer heat thereto. Insome embodiments, the heat exchanger 60 is configured to raise the inlettemperature of the torrefaction gas stream 34 into the heat exchanger 60from about 500° F.±100° F. to an outlet temperature of about 700°F.±150° F. In doing so the temperature of the other isolated gas stream35 in the heat exchanger 60 is necessarily lowered before exiting theheat exchanger 60 via an outlet conduit 66. The temperature of the otherisolated gas stream 35, however, is still sufficiently hot to be usefulin other processes, such as, for example, drying the biomass particlesprior to entry in the biomass torrefaction system 10. Accordingly, insome embodiments, the gas stream 35 discharged from the heat exchanger60 via the outlet conduit 66 may be routed to a dryer system 70 (FIG. 2)or other device, as represented by the arrow labeled 68. In someembodiments, the discharged gas stream 35 may be routed back to theinlet of the heat exchanger 60 and blended with other heated gas havinga higher temperature, such as, for example, a remote burner, to regulatethe inlet temperature of the heat exchanger 60 to a desired level or tofall within a desired temperature range.

Although the illustrated embodiment of the heat source 30 of FIG. 1 isshown as a gas-to-gas heat exchanger 60, it is appreciated that othervarious heat sources 30 may be provided. For example, in someembodiments, an electric immersion-type heat source may be providedwithin the path of the gas stream 34 of the biomass torrefaction system10. In other embodiments, low oxygen burners may be directed directlyinto the system 10 to heat the gas stream 34 without significantlyincreasing the oxygen level within the system 10. Irrespective of theheat source 30, however, it is beneficial to isolate the gas stream 34in a recirculating manner to facilitate maintenance of a low leveloxygen environment within the reactor drum 12 that is conducive totorrefying biomass particles.

At the downstream end of the reactor drum 12, there is provided aseparator hopper 38 for collecting torrefied biomass particles (e.g.,torrefied wood chips, torrefied giant cane chips, other torrefiedcellulosic biomass) as the particles exit the reactor drum 12. Theseparticles are then fed mechanically and/or under the force of gravitytowards an outlet 40 for collection. One or more airlock devices 42 arecoupled to the outlet 40 for substantially preventing oxygen frominfiltrating the system 10 as the torrefied particles are withdrawn fromthe system 10. Smaller particles (e.g., torrefied wood fines, torrefiedgiant cane fines, other torrefied cellulosic biomass) which may passthrough the separator hopper 38 can be filtered and removed from the gasstream 34 by a filtering device 44, such as, for example a cyclonic typefiltering device. One or more additional airlock devices 46 may becoupled to a secondary outlet 48 for removing the filtered material fromthe system 10 without introducing significant amounts of oxygen into thesystem 10. In some embodiments, a chamber or space between a pair ofsequentially aligned airlocks 42, 46 may be coupled to an inert orsemi-inert gas source for selective purging of the chamber or space, asrepresented by the arrows labeled 43, 47 (FIG. 2). In some embodiments,the torrefaction system 10 may include a cyclonic type filtering devicein lieu of a hopper 38 to separate and/or filter torrefied biomassparticles from the gas stream 34. In some embodiments, the torrefactionsystem 10 may include one or more pneumatic discharge devices (notshown) to discharge torrefied biomass particles from the torrefactionsystem 10.

As previously described, the gas stream 34 is drawn or forced throughthe reactor drum 12 and returned to the heat source 30 (after separatingtorrefied particles, chips, fines, dust and/or any debris) under theinfluence of the fan device 32. While the substantial majority of thegas is recirculated, some gas may be diverted to exhaust ducting 50. Thegas exhausted through the exhaust ducting 50 can be used elsewhere inthe process or another process, as represented by the arrow labeled 52.For instance, the exhaust gas may be used as fuel to generate heat toaid the heat source 30 in increasing the temperature of the gas stream34. The exhaust ducting 50 can include a variable position damper 54which may be used to balance the pressure inside the reactor drum 12from slightly negative to slightly positive. Depending on the setting,this can be used to inhibit oxygen from entering the system 10.

FIG. 2 shows a schematic of an integrated biomass processing system 11according to one example embodiment. The integrated biomass processingsystem 11 includes, among other things, the biomass torrefaction system10 described above and a dryer system 70 which is configured to drybiomass particles prior to introduction into the torrefaction system 10.In some embodiments, the biomass torrefaction system 10 is configured toreceive biomass particles having a moisture content reduced to belowtwenty percent moisture content, wet-weight basis by the dryer system70. In some embodiments, the biomass particles may be wood chips havingan average particle size between about 1/16 cubic inch and about onecubic inch and having an intial moisture content above forty percentmoisture content, wet-weight basis. In some embodiments, the biomassparticles may have a substantially consistent size (less than tenpercent difference), and in other embodiments, the size of the particlesmay vary by ten percent, twenty percent, thirty percent or more.

According to the illustrated embodiment of FIG. 2, the dryer system 70includes a rotary drum 71 which is supported so as to rotate about itslongitudinal axis 72. The dryer system 70 further includes an inlet 74for receiving biomass particles that are to be processed, as representedby the arrow labeled 75. The biomass particles may be fed to the inlet74 via a conveyor or other conventional material transport mechanism.

The dryer system 70 is coupled to a burner 76 which is configured tofeed a heated gas stream via ducting 77 through the rotary drum 71 andintermittingly carry biomass particles through the drum 71 as itrotates. The heated gas stream simultaneously dries the biomassparticles as the gas stream propels the particles through the rotarydrum 71. The burner 76 may be configured to burn bark, hogged fuel orother fuels to heat the gas stream fed to the dryer system 70. The gasstream entering the dryer system 70 may also be supplemented or blendedwith other gas streams of the integrated biomass processing system 11 asdescribed in further detail elsewhere.

At the downstream end of the rotary drum 71, there is provided aseparator hopper 78 for collecting dried biomass particles (e.g., driedwood chips, dried giant cane chips, other dried cellulosic biomass) asthe particles exit the rotary drum 71. These particles are then fedmechanically and/or under the force of gravity towards an outlet 79 forcollection for subsequent use or packaging. Smaller particles and dust(e.g., dried wood fines, dried giant cane fines, other dried cellulosicbiomass) which may pass through the separator hopper 78 are filtered andremoved from the gas stream by a filtering device 80, such as, forexample a cyclonic type filtering device. These particles are fedtowards a secondary outlet 81 for subsequent use or packaging. In someembodiments, the dryer system 70 may include a cyclonic type filteringdevice in lieu of a hopper 78 to separate and/or filter dried biomassparticles from the gas stream. In some embodiments, the dryer system 70may include one or more pneumatic discharge devices (not shown) todischarge dried biomass particles from the drier system 70.

A fan device 92 may be provided to draw or force the gas stream throughthe rotary drum 71 and to route exhaust gas from the rotary drum 71toward environment emission control equipment 82 to process the exhaustof the dryer system 70 before release to the environment or to othersystems, as represented by the arrow labeled 83. As an example, theemission control equipment 82 may include a wet electrostaticprecipitator (WESP) to facilitate the removal of sub-micron sized solidparticles and liquid droplets from the exhaust gas stream. The emissioncontrol equipment 82 may further include a regenerative thermal oxidizer(RTO) to destroy air toxics and volatile organic compounds (VOCs) thatmay be present in the exhaust gas. In some embodiments, an RTO may beprovided which uses natural gas to heat the exhaust gasses to about1500° F. where VOCs are oxidized. In other embodiments, torrefieroff-gasses may be used for heating of the RTO which may significantlyreduce the operating cost of the RTO since natural gas is otherwise asignificant cost in operating such equipment.

At least a portion of the exhaust from the dryer system 70 may be routedor recycled back towards the inlet 74 of the rotary drum 71 and combinedwith the heated gas stream from the burner 76 to dry the biomassparticles which are continuously fed into the rotary drum 71, asrepresented by the arrows labeled 84. Additional gases from the outletof the heat exchanger 60 of the torrefaction system 10 may also becombined with the exhaust gases from the dryer system 70 for cleansingprior to discharge into the environment and/or for introduction backinto the dryer system 70, as represented by the arrows labeled 85.

According to the illustrated embodiment of FIG. 2, the dried biomassparticles (e.g., dried wood chips and fines) may be routed to anotherlocation for subsequent processing, storage or packaging of the driedbiomass particles as a standalone commodity, as represented by the arrowlabeled 86. A portion or the entire supply of the dried biomassparticles may be routed to the torrefaction system 10 for subsequentprocessing, as indicated by the arrow labeled 87.

As can be appreciated from FIG. 2, the dried biomass particles generatedvia the dryer system 70 may serve as input material for the torrefactionsystem 10. In some embodiments, the dried biomass particles may have anaverage moisture content below twenty percent moisture content,wet-weight basis when entering the torrefaction system 10. In otherembodiments, the average moisture content of the dried biomass particlesmay be between about five percent moisture content, wet-weight basis andabout fifteen percent moisture content, wet-weight basis. In still otherembodiments, the average moisture content of the dried biomass particlesmay be greater than twenty percent moisture content, wet-weight basis.

Although the dryer system 70 is illustrated as rotary drum type dryersystem, such as those designed and marketed by Teal Sales Incorporated,the assignee of the present application, it is appreciated that otherdryer systems may be utilized in connection with embodiments of thepresent invention, including, for example, kilns having rotary screw andconveyor bed type conveyance mechanisms. Accordingly, embodiments of thebiomass processing systems described herein are not limited to thespecific dryer systems illustrated, but may incorporate a wide range ofconventional dryer systems.

With continued reference to FIG. 2, the heat source 30 is shown as agas-to-gas heat exchanger 60 which is configured to receive a heated gasstream from the burner 76, as indicated by the arrow labeled 88. Theheated gas stream entering the heat exchanger 60 may be blended withgases from an output of the heat exchanger 60, as represented by thearrows labeled 90, to regulate the input temperature of the heated gasstream entering the heat exchanger 60. In some embodiments, the inlettemperature of the gas stream entering the heat exchanger may be betweenabout 600° F. and about 1400° F., and in some embodiments, the inlettemperature of the gas stream entering the heat exchanger 60 may bebetween about 800° F. and about 1000° F. The recirculating gas stream ofthe torrefaction system 10 passes through the heat exchanger 60 and isheated, according to some embodiments, to a reactor drum inlettemperature of at least 500° F. After passing through the reactor drum12 the heated gas stream has a reactor drum outlet temperature of atleast 400° F. Consequently, the biomass particles which are passedthrough the torrefaction reactor drum 12 during operation are directlysubjected to a heated gas stream having a temperature at least 400° F.over the entire length of the reactor drum 12. In some embodiments, thereactor drum inlet temperature is about 700° F.±150° F. and the reactordrum outlet temperature is about 500° F.±100° F. The reactor drum inletand reactor drum outlet temperatures of the heated gas stream may bemonitored with appropriate temperature sensors and controlled via ageneric or cascaded control loop to maintain the temperature gradientthrough the reactor drum at a desired level during operation.

Exhaust gases from the torrefaction process, which include hydrocarboncompounds boiled out of the biomass particles, water vapor and anyambient air that leaks into the system may be routed, according to someembodiments, to the burner 76 for combustion, as indicated by the arrowlabeled 91. In this manner, energy contained in the exhaust gasses canbe utilized to heat a heat transfer medium for use in the heat exchanger60 to maintain the heated gas stream 34 flowing through the reactor drum12 at a desired elevated inlet temperature. Again, in some embodiments,the reactor drum inlet temperature may be about 700° F.±150° F. and thereactor drum outlet temperature may be about 500° F.±100° F. The reactordrum temperature gradient may be controlled through a cascaded controlloop which sets the reactor drum inlet temperature. The reactor druminlet temperature may be controlled, for example, by varying the amountof heated gas fed to the heat exchanger 60 from the burner 76. In someembodiments, the burner 76 may be configured to burn bark, hogged fuelor other fuel to heat the gas stream 35 fed through the heat exchanger60. Again heating of this gas stream 35 may be supplemented with thecombustion of exhaust gases from the torrefaction system 10, asrepresented by the arrow labeled 91.

FIGS. 3 through 8 illustrate a biomass torrefaction system 110 accordingto another example embodiment similar to the biomass torrefactionsystems 10 described earlier, but with additional structural details anda different example heat source 130. The system 110 includes a reactordrum 112 which is supported on a structural frame 114 to rotate about ahorizontal axis of rotation 116. The reactor drum 112 is driven by adrive motor 118 which may be electrically coupled to a control systemfor selectively controlling the rotation of the reactor drum 112 andoptionally adjusting the speed thereof. The control system includes acontrol panel 120 with appropriate controls (switches, dials, gauges,etc.) for selectively controlling and monitoring the system 110. Othergauges and controls (e.g., sensors, valves, etc.) may be remotelylocated and coupled to specific components of the system for monitoringand control purposes.

The system 110 further includes an inlet 122 in the form of a chute forreceiving biomass particles that are to be processed, as represented bythe arrow labeled 124. An airlock or dual airlock 126 with optionalinert or semi-inert gas purging or similar device is coupled to theinlet 122 to substantially prevent oxygen from entering the system 110when biomass particles are input. The biomass particles may be fed tothe inlet 122 via a conveyor or other conventional material transportmechanism. The rate of introduction of biomass particles may bemonitored and controlled to optimize or tailor characteristics of theresultant torrefied biomass particles. Stairs 128 or other accessdevices may be provided for a user to access the inlet 122 and othercomponents of the system 110 for monitoring, maintenance and otherpurposes.

The system 110 also includes a heat source 130 disposed upstream of thereactor drum 112 for supplying heat to a gas stream that is generated inthe system 110 by a fan device 132, which may be, for example, aninduced draft fan device or a forced draft fan device. The fan device132 is driven by a drive motor 134 to draw or force gas through thereactor drum 112 and circulate it back to the heat source 130 to bereheated and supplied to the reactor drum 112 in a recirculating manner.Gas ducts 136 are appropriately sized and coupled to at least thereactor drum 112, heat source 130 and fan device 132 for this purpose.

At the downstream end of the reactor drum 112, there is provided aseparator hopper 138 for separating torrefied biomass particles from thegas stream as the particles exit the reactor drum 112. These particlesare then fed mechanically and/or under the force of gravity towards anoutlet 140 for collection for subsequent use or packaging. An airlockdevice 142 is coupled to the outlet 140 for substantially preventingoxygen from infiltrating the system 110 as the torrefied particles arewithdrawn. Smaller particles and dust which may pass through theseparator hopper 138 are filtered and removed from the gas stream by afiltering device 144, such as, for example a cyclonic type filteringdevice. Another airlock device 146 may be coupled to a secondary outlet148 for removing the filtered material from the system 110 withoutintroducing significant amounts of oxygen to enter the system 110. Insome embodiments, the system 110 may include a cyclonic type filteringdevice in lieu of a hopper 138 to separate and/or filter torrefiedbiomass particles from the gas stream passing through the reactor drum112. In some embodiments, the system 110 may include one or morepneumatic discharge devices (not shown) to discharge torrefied biomassparticles from the system 110.

As previously described, the gas stream is drawn or forced through thereactor drum 112 and returned to the heat source 130 (after separatingtorrefied particles, dust and any debris) under the influence of the fandevice 132. While the substantial majority of the gas is recirculated tothe reactor drum 112, some gas is diverted to an exhaust stack 150. Thegas exhausted through the stack 150 can be recaptured for use elsewherein the process or another process, such as, for example, use as fuel togenerate heat. The stack 150 can include a variable position damper 152which may be used to balance the pressure inside the reactor drum 112from slightly negative to slightly positive. Depending on the setting,this can be used to inhibit oxygen from entering the system 110.

Further details of the reactor drum 112 will now be described withreference to FIGS. 7 and 8. As shown in the illustrated embodiment, thereactor drum 112 is supported in a horizontal orientation on a number ofrollers 160. The rollers 160 contact the drum 112 along bearing tracks162 that are secured to a circumference of the drum 112. The diameter ofthe drum 112 may be three, four, five feet or more and may be configuredto receive and process over fifty tons of torrefied biomass particlesper hour.

The drive motor 118 is coupled to a drive belt or chain 164 andcontrolled via the control system to selectively rotate the drum 112 atvarious speeds, such as, for example, about 3 rpm or more or less. Highprecision seals 166 are disposed between the rotating drum 112 andstatic components to prevent the infiltration of oxygen into the system.In this manner, the seals 166 and other features of the system are ableto maintain the gas stream at a consistent low level of oxygen bycreating a substantially sealed vessel.

Within the reactor drum 112, there are a number of lifting flights 170spaced circumferentially at each of a plurality of locations along alongitudinal length thereof. The density of the lifting flights 170 maybe designed to suit various needs of the system 110 and may be dependenton a number of interrelated factors, such as, for example, the speed ofrotation of the reactor drum 112, the rate of material fed into thesystem 110, and the speed of the fan device 132 or strength of theheated gas stream passing through the reactor drum 112. The flights 170are configured to lift biomass particles as the reactor drum 112 rotatesin the direction indicated by arrow 172 and then direct and shower thebiomass particles into the gas stream to be intermittingly carried alongthe length of the reactor drum 112 predominately by the kinetic energyof the gas stream and simultaneously torrefied. This is advantageous inthat the transport mechanism for the biomass particles provides a highlyefficient medium for transferring heat to the particles directly.Accordingly, large volumes of biomass particles can be processed by asystem with reduced energy demands. In addition, the throughput or rateof torrefied biomass particles (tons/hour) may be relatively greaterwhen compared to conventional torrefaction systems of generallycomparable size.

The biomass particles reside in the drum 112 for a period of time andthen are subsequently discharged into the separator hopper 138 or otherseparating device and routed in the direction indicated by the arrowlabeled 174 for further handling. A predominate or substantial portionof the gas stream is routed in the direction indicated by the arrowlabeled 176 and recirculated, heated and reintroduced into the reactordrum 112 as indicated by the arrow labeled 178.

The system 110 thus enables a continuous torrefaction process thatinvolves the introduction of biomass particles into a rotating reactordrum 112 via an airlock or airlocks 126 to maintain a low oxygen levelinside the torrefaction system 110 which is conducive to torrefyingbiomass particles. The particles are conveyed through the drum 112 bythe kinetic energy of a heated gas stream that is generated by creatingan induced draft of forced draft via a fan device 132 connected by aduct 136 to the outlet of the drum 112. There is also a heat source 130upstream of the drum 112, such as, for example, an electricalimmersion-type duct heater (FIG. 3) or a gas-to-gas heat exchanger (FIG.1). The fan device 132 draws or forces gas across or through the heatsource 130 and through the drum 112. Beneficial to the viability of theprocess is the recirculation of gas exiting the drum 112 back to theheat source 130 for reheating. Also beneficial to the viability of theprocess is the ability of the heated gas stream to directly heat thebiomass particles under a low oxygen environment as the gas streamsimultaneously transports the biomass particles intermittently throughthe reactor drum 112, as discussed in more detail elsewhere.

There is of course a certain flow of gas that is discharged from thesystem 110 (whether to the external environment or another related orunrelated process component) which is substantially equal to the sum ofthe gases being driven off of the biomass particles due to heating(including water evaporation) and any leakage that may enter the system110.

The interior of the drum 112 contains specialized lifting- andfall-distance-control flights 170 that lift and shower the particles asthe drum 112 rotates thereby exposing the particles to the heated gasstream causing moisture within the particles to be evaporated. As theparticles shower within the drum 112 the moving gas within the drum 112causes them to be conveyed forward. It generally takes a number ofrotations of the drum 112 to provide enough forward progress of theparticles to gain passage through the length of the drum 112. Theshowering and conveying process within the drum 112 also classifies theparticles. Lighter, smaller particles pass through the drum 112 fasterthan heavier, larger particles. This allows large particles to remain inthe drum 112 for a relatively longer residence time and creates a moreuniform end product (i.e., large and small particles may be processedtogether to have similar end characteristics despite differences in massand volume). For example, in some embodiments, particle size may varywithin a particular run of torrefied biomass particles by ten, twenty orthirty percent or more while the energy density and moisturecharacteristics of the particles are maintained relatively consistentirrespective of particle size. In some embodiments, the flights 170 maybe designed to vary with respect to location and/or flight density indifferent embodiments to affect the residence time of the biomassparticles within the reactor drum 112.

When using the system 110 to torrefy biomass particles the heat source130 is responsible for adding heat to a recirculating gas system withinthe system 110. The heated gas stream within this recirculating gassystem in turn directly heats the biomass particles as they are conveyedthrough the system 110. In this manner, the heated gas stream directlyheats and transports the biomass particles simultaneously. This isadvantageous in that the transport mechanism for the biomass particlesprovides a highly efficient medium for transferring heat to theparticles directly. Accordingly, large volumes of biomass particles canbe processed by a system with reduced energy demands. In addition, thethroughput or rate of torrefied biomass particles (tons/hour) may berelatively greater when compared to conventional torrefaction systems ofgenerally comparable size. This advantageously enables the systemsdescribed herein to be implemented in a particularly commerciallyfeasible manner.

Elements of the heat source 130 can provide heat by any readilyavailable energy source. In some embodiments, for example, direct heatmay be applied to the gas stream by an electric element (e.g.,electrical immersion-type duct heater 130). In other embodiments, heatmay be provided to the gas stream through a gas-to-gas heat exchanger 60(FIGS. 1 and 2) coupled to a combustion and/or waste heat system (e.g.,burner 76 of FIGS. 1 and 2). In another embodiment, low oxygen burnersmay be directed directly into the system 110 to heat the gas streamwithout significantly increasing the oxygen level within the system 110.In some embodiments, exhaust gas that is discharged from the stack 150may be used as part of the process heating fuel. Irrespective of theheat source 130, very little additional oxygen is added to the system110 throughout the heating portion of the process.

The torrefaction systems and processes are based on a heat and energybalance that balances the energy required with the process rate, heatingsource and required residence time. Embodiments of the torrefactionsystems and methods described herein are particularly well suited tomanipulate and control these factors and provide systems and methodsthat are readily scalable to meet various industry needs.

For instance, residence time of the particles within the drum 112 may becontrolled by various design and process factors. For example, the speedand size of the fan device 132 may be selected to adjust the velocity ofthe circulating heated gas within the drum 112. In addition, the speedand volume of the heated gas stream can also be adjusted by a fan inletdamper of the fan device 132. As another example, the rotation speed ofthe drum 112 may be set higher of lower to adjust the rate of thelifting and showering effect within the drum 112 thus creating more orless time in which the particles are in suspension. Further, since theflights 170 may be designed to work over a wide range of rotationalspeeds, the drum 112 rotational speed can be selectively adjusted byappropriate controls (such as a variable speed drive motor) to adjustthe residence time. Also, the density of the flights 170 within the drum112 can be used to change the flow conditions inside the drum 112 givingan individual design an inherent shorter of longer residence time. Stillfurther, the size and shape of the flights 170 can be altered to meetthe needs of the material processed and create a more or less pronouncedshowering effect, thereby impacting the residence time in the drum 112.

In some embodiments, the flights 170 may be secured to the drum 112 in aparticular density and arrangement to optimize or tailor characteristicsof the resultant torrefied biomass particles. The length of the drum 112can also be varied in initial design to create more or less residencetime. In addition, particle loading conditions can be varied to createmore of less resistance to the gas stream within the drum 112, thusaffecting residence time. For example, in some embodiments, a relativelygreater volumetric flow rate of biomass particles may be set to crowdthe interior of the drum 112 and slow the progression of the particlesthrough the drum 112. Conversely, a relatively smaller volumetric flowrate of biomass particles may be set to reduce crowding in the interiorof the drum 112 and speed the progression of biomass particles throughthe drum 112.

The oxygen level inside the drum 112 may likewise be controlled byvarious design and process factors. For example, the mechanical designof the particle inlet can be selected to include, for example, anairlock, a gas-purged double airlock, screw mechanisms or the like, witheach mechanism having a different level of ability to prevent theinfiltration of oxygen. Preferably, the amount of oxygen that enters thesystem 110 with the particles is minimized, but is likely to vary withdesign according to particle size and/or desired production rate of theprocessed biomass. In addition, the incoming moisture content of theparticles can be varied to control oxygen level. During processing, theresulting evaporated water partially displaces oxygen within the system110, and thus the level of moisture can be varied to suit productionrequirements (e.g., less initial moisture means less energy required totorrefy the particles, and more initial moisture results in less oxygenin the system). Still further, it is recognized that there is a netaddition of gas to the system as volatiles and moisture are evaporatedfrom the particles. As previously described, this excess gas may beexhausted from the system 110 via a stack 150 and may, according to someembodiments, be recaptured for use elsewhere in the process or anotherprocess, such as, for example, use as fuel to generate heat. The stack150 can include a variable position damper 152 which may be used tobalance the pressure inside the drum 112 from slightly negative toslightly positive. Depending on the setting of the damper 152, this canbe used to inhibit oxygen from entering the system 110.

In some embodiments, many of the various operational parametersdiscussed above as well as other operational parameters may be adjusted(manually or automatically) during operation. In other embodiments,operational parameters may be established prior to operation.Irrespective of the particular control scheme, the ability toindependently control various operational parameters of the systemsdescribed herein provide for particularly versatile biomass torrefactionsystems and methods that are adaptable to changing conditions, such as,for example, the moisture content of the biomass particles selected tobe processed and a desired energy density of resultant torrefied biomassparticles which may vary.

The system 110 may also be outfitted with precision seals 166 atrotating to static connections and other low leakage connections andcomponents to provide a particularly well sealed vessel to maintainconsistent low levels of oxygen within the system 110.

FIGS. 9 through 11 illustrate one example embodiment of a precision sealassembly 266 that may be used to substantially eliminate theinfiltration of oxygen of the surrounding environment into the reactordrum 212 at a rotational interface. As shown best in FIG. 10, the sealassembly 266 may include rigid flange structures 270 which are coupledto a flange 268 of the reactor drum 212 to rotate in unison therewith.The flange structures 270 may extend toward stationary flange structures272 positioned upstream of the drum 212 with respect to the flowdirection F. A gap or space between the stationary flange structures 272and the rotating flange structures 270 may be spanned by seal elements274 to define an internal chamber 276. This internal chamber 276 may bepurged intermittingly with inert or semi-inert gas to maintain an inertor semi-inert gas barrier between an environment external to the sealassembly 266 and an internal environment of the reactor drum 212.

The seal elements 274 may include internal stiffeners to providesufficient rigidity to maintain the seal elements 274 in sealing contactwith the rotating flange structures 270 as the drum 212 rotates duringoperation about the rotational axis 216. Additional biasing elements 280may also be provided to urge one or more of the seal elements 274 intofirm contact with the rotating flange structures 270. In the illustratedembodiment, the biasing elements 280 are shown as overlapping springelements extending from the stationary flange structures 272 positionedupstream of the reactor drum 212 to a seal element 274 overlying one ofthe rotating flange structures 270. As shown in FIG. 11, the sealelements 274 may be spliced together in the manner shown to preventfraying of the seal elements 274 as the reactor drum 212 and flangestructures 270 rotate in the direction R during operation.

Although each of the flange structures 270, 272 are illustrated asL-shaped structural members, it is appreciated that the size and shapeof the flange structures 270, 272 may vary significantly. Irrespectiveof size and shape, however, it is beneficial, according to someembodiments, to provide an isolated internal chamber 276 which may beselectively purged as needed (e.g., during system startup, shutdown orfault conditions) with inert or semi-inert gas to assist in maintainingthe internal environment within the reactor drum 212 at a consistent lowlevel of oxygen. In addition, irrespective of the size, shape andconfiguration of the elements of the seal assembly 266, a redundant sealinterface is beneficial to help minimize leakage into the internalenvironment.

It is further appreciated that other seals and sealing devices (e.g.,airlocks or dual airlocks) may be provided at other potential leakpoints in the system, including, for example, at the biomass particleinlets and outlets. In addition, substantially sealed chambers may alsobe formed in these locations between the torrefaction system and theexternal environment. These chambers may be coupled to inert orsemi-inert gas sources for intermittent purging of the chambers withinert or semi-inert gas, such as, for example, at system startup,shutdown or during fault conditions. Purging these chambers mayadvantageously ensure that no or very little oxygen from the surroundingenvironment infiltrates the recirculating gas of the torrefactionsystem. In some embodiments, the system may be equipped with dual infeedand discharge airlocks that are arranged in series with inert orsemi-inert gas purging enabled between the airlocks.

Various safety devices may also be incorporated into the torrefactionsystems to enhance operational safety. For instance, the systems may beequipped with vents that will rupture or open should a minor explosionor deflagration occur of sufficient magnitude to potentially causeequipment damage. As another example, spark detection and extinguishmentsystems may also be integrated into the torrefaction systems, such as,for example, spark detection and extinguishment systems and componentsmarketed by GreCon, Inc. headquartered in Tigard, Oreg. In addition,system operational characteristics may be monitored, for example, byvarious sensors (e.g., temperature, pressure, oxygen, etc.), and theobtained operational data may be used to adjust and control the systemas needed to enhance safety or to optimize the torrefaction process. Insome embodiments, real time mass spectroscopy may also be used toidentify compounds in the gas streams and to adjust or control thesystem as needed to enhance safety or to optimize the torrefactionprocess.

In some embodiments, steam from a separate boiler of a steam plant 93(FIG. 2) which is fired by the off gas of the reactor drum 12 (asrepresented by the arrow labeled 94) or another fuel or heat source maybe injected into the system 10 (as represented by the arrow labeled 95)to further control oxygen in the process or as a safety smothering andcooling stream and also may be used as an inert or semi-inert purge gasin the process. In addition, using steam as part of the process gaswhich passes through the reactor drum 12 may also improve heat transferto the biomass particles. In some embodiments, the boiler may be heatedby off gas routed thereto by ducting 96 coupled to the reactor drum 12.In other embodiments, the boiler may be heated by the burner 76 oranother heat source. In some embodiments, upon a fault condition, steammay be introduced into the reactor drum 12 in sufficient quantities forsmothering and cooling purposes. In this manner, operational safety ofthe torrefaction system 10 may be enhanced,

Overall, by knowing the processes by which heat, residence time andoxygen levels are controlled and by having the flexibility throughinitial design and the numerous process variables described herein,embodiments of the biomass torrefaction systems and methods can be setup to accommodate a variety of biomass feed stocks in a variety of localconditions and provide the flexibility and control needed to achieveconsistent torrefaction results. In some embodiments, for example, thetorrefaction systems and methods may be configured to torrefy biomassparticles in the form of wood chips at a minimum rate of one ton oftorrefied biomass particles per hour with the resultant torrefiedbiomass particles having an energy density of at least 20 GJ/ton.

The torrefaction systems and methods described herein are particularlywell suited to provide a continuous torrefaction process that has manybenefits over conventional torrefaction systems, and in particular,batch systems and methods which require batch processing of biomassparticles in a furnace, kiln or other similar device. The continuousnature of the torrefaction systems and methods described herein enable,among other things, relatively higher production rates. In addition, theefficiency with which biomass particles may be processed with thesystems and methods enable high material throughput at relatively lowerenergy demands.

Although embodiments of the torrefaction systems and methods describedherein are illustrated in the figures as including reactor drums whichrotate about a horizontally aligned axis of rotation, it is appreciatedthat in some embodiments, the axis of rotation may be inclined. In suchembodiments, gravity may play a significant role in transporting thebiomass particles through the reactor drum. In addition, althoughembodiments of the torrefaction systems and methods are described hereinas involving a heated gas stream passing through the reactor drum tocarry or transport the biomass particles while simultaneouslytransferring heat to the biomass particles to torrefy them, it isappreciated that in some embodiments the biomass particles may betransported by alternate mechanisms (e.g., gravity, screw devices,conveyor devices, etc.) and subjected to a counter-flowing heated gasstream within the reactor drum to torrefy the biomass particles.

Moreover, the various embodiments described above can be combined toprovide further embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled.

The invention claimed is:
 1. A biomass torrefaction system, comprising:an inlet to receive biomass particles; a reactor drum configured torotate about an axis of rotation, the reactor drum having a plurality offlights positioned therein; a heat source to heat gas contained in thesystem to a temperature sufficient to torrefy the biomass particlesduring operation; a fan device coupled to the system to create, when thesystem is in operation, a heated gas stream through the reactor drumsufficient to intermittently transport the biomass particles along alongitudinal length of the reactor drum as the biomass particles arelifted by the flights and showered through the heated gas stream as thereactor drum rotates; and a duct system to recirculate at least aportion of gas exiting the reactor drum back to the heat source toreheat the gas for reintroduction into the reactor drum.
 2. The biomasstorrefaction system of claim 1 wherein the heated gas stream directlyheats the biomass particles as the gas stream intermittently transportsthe biomass particles through the reactor drum.
 3. The biomasstorrefaction system of claim 1 wherein the plurality of flights areconfigured to regulate movement of the biomass particles through thereactor drum, thereby influencing a retention time of the biomassparticles within the reactor drum.
 4. The biomass torrefaction system ofclaim 3 wherein the plurality of flights include flights spaced aroundan inner circumference of the reactor drum in regular or irregularintervals and in at least three locations along the longitudinal lengthof the reactor drum.
 5. The biomass torrefaction system of claim 1wherein the plurality of flights interoperate with the heated gas streamto classify the biomass particles according to particle density bymoving relatively denser particles with respect to similarly sizedparticles through the reactor drum more slowly.
 6. The biomasstorrefaction system of claim 1, further comprising: ducting to dispelexhaust gas from the system; control valves; and dampers, the controlvalves and dampers positioned to regulate a pressure level within thesystem to inhibit the infiltration of oxygen into the system whileenabling exhaust gas to exit the system.
 7. The biomass torrefactionsystem of claim 1, further comprising: ducting to route exhaust gas fromthe system to a remote device for use of the exhaust gas in an auxiliaryor supplemental process.
 8. The biomass torrefaction system of claim 7wherein the remote device is a burner configured to utilize the exhaustgas to generate a heated medium for supplying heat via a heat exchangerto the gas which passes through the reactor drum during operation. 9.The biomass torrefaction system of claim 1, further comprising: at leastone airlock coupled between the inlet and the reactor drum to limit theamount of oxygen entering the system when receiving the biomassparticles; and at least one seal mechanism between the reactor drum andadjacent structures, the seal mechanism including a chamber between thereactor drum and an external environment and the seal mechanism coupledto an inert or semi-inert gas source for selective purging of thechamber during operation.
 10. The biomass torrefaction system of claim1, further comprising: a steam plant coupled to the reactor drum tointroduce steam into the reactor drum and assist in the torrefaction ofthe biomass particles.
 11. The biomass torrefaction system of claim 1,further comprising: a control system configured to independently controla plurality of operational parameters to regulate a torrefaction processof the biomass particles, the operational parameters including at leastone of a reactor inlet temperature, a reactor outlet temperature, anaverage residence time, oxygen content of the heated gas stream and gasflow characteristics.
 12. The biomass torrefaction system of claim 11wherein the control system includes sensors for monitoring at least someof the operational parameters and the control system is configured tocontinuously or intermittingly adjust at least some of the operationalparameters during operation to optimize the torrefaction process ortailor characteristics of the resultant torrefied biomass particles. 13.The biomass torrefaction system of claim 1 wherein the reactor drum isat least five feet in diameter and the system is configured to torrefybiomass particles at a minimum rate of one ton of torrefied biomassparticles per hour, the torrefied biomass particles having an energydensity of at least 20 GJ/ton.
 14. A method of biomass torrefactioncomprising: rotating a reactor drum about an axis of rotation, thereactor drum having a plurality of flights positioned therein;generating a heated gas stream through the reactor drum sufficient tointermittently transport biomass particles along a longitudinal lengthof the reactor drum and simultaneously torrefy the biomass particles asthe biomass particles are lifted by the flights and showered through theheated gas stream as the reactor drum rotates; and recirculating aportion of gas exiting the reactor drum back to an inlet of the reactordrum for torrefying biomass particles within the reactor drum.
 15. Themethod of biomass torrefaction of claim 14, further comprising:selectively varying at least some of a plurality of operationalparameters to tailor characteristics of the resultant torrefied biomassparticles, the operational parameters including at least one of a speedof the heated gas stream through the reactor drum, a volumetric flowrate of the heated gas stream through the reactor drum, a temperature ofthe heated gas stream through the reactor, a pressure level within thereactor drum, a speed of the rotation of the reactor drum, oxygencontent of the heated gas stream, a moisture content of the biomassparticles and a rate of introduction of the biomass particles into thereactor drum.
 16. The method of biomass torrefaction of claim 14,further comprising: routing exhaust gas to a device remote from thereactor drum for use of the exhaust gas in an auxiliary or supplementalprocess.
 17. The method of biomass torrefaction of claim 14, furthercomprising: passing biomass particles through the reactor drum at aminimum rate of one ton per hour, the biomass particles having an energydensity of at least 20 GJ/ton after being torrefied within the reactordrum.
 18. The method of biomass torrefaction of claim 14, furthercomprising: drying the biomass particles in a rotary type dryer systemprior to introduction in the reactor drum.
 19. The method of biomasstorrefaction of claim 14, further comprising: venting the reactor drumupon a fault condition.
 20. The method of biomass torrefaction of claim14, further comprising: introducing steam into the reactor drum toassist in the torrefaction of the biomass particles.